Image pick-up tube with a photosensitive transmission secondary electron multiplication layer



IMAGE PICK--UP SECONDARY M. Filed Feb. 12, 1968 5,535,574 "FIVETRANSMISSION. 'UIGATION LAYER 2 Sheets-Sheet 1 FIG; I

INVENTOR H/mu 0 MHE D H ATTORNEY Oct. 20, 1970 wimuo MAEQA IMAGE PICK-UPTUB WITH 32 PHOTOSENSITIVE TRANSMISSION SECONDARY ELECTRONMULTIPLICATION LAYER 2 Sheets-Sheet 2 Filed Feb. 12, 1968 INVENTORHRRLLD IIHEDH ATTORNEYJ United States Patent Ofice Patented Oct. 20,1970 3,535,574 IMAGE PICK-UP TUBE WITH A PHOTOSENSITIV E TRANSMISSIONSECONDARY ELECTRON MUL- TIPLICATION LAYER Haruo Maeda, Tokyo, Japan,assignor to Matsushita Electric Industrial C0., Ltd., Osaka, Japan, acorporation of Japan Filed Feb. 12, 1968, Ser. No. 704,882 Claimspriority, application Japan, Feb. 24, 1967, 42/ 12,202; May 29, 1967,42/334,767; May 30, 1967, 42/35,044; July 25, 1967, 42/48,283 Int. Cl.H013 31/48, 43/02; H011 15/00 US. Cl. 313-65 Claims ABSTRACT OF THEDISCLOSURE An image pick-up tube comprising a target, wherein thetransmission secondary electron multipling porous layer hasphotoconductivity and the transmission secondary electron multiplicationis controlled by the impedance change of said photoconductive porouslayer corresponding to the incident light of an image.

This invention relates to a novel image pick-up device which utilizesthe transmission secondary electron multiplication effect of a porousphotoconductive material. This invention resembles a known vidicon inthat a photoconductive material is used as a photosensitive surface, butit is quite different in its fundamental operation.

In a vidicon type image pick-up tube, a photoconductive evaporated filmlayer provided as a photosensitive surface exhibits a resistancedistribution corresponding to an intensity distribution of the lightimage projected thereon and a television image signal is obtained bydischarging a charge from a NESA film through a load resistance, saidcharge being accumulated during a scanning period in the distributedcapacitance considered to exist between the scanning surface of thephotoconductive film and the NESA conductive layer of the substrate whenthe surface of said photoconductive layer is scanned by a low velocityelectron beam. Thus, in a conventional vidicon, the combination of asensitive photoconductive film and a low velocity scanning electron beamusually below several hundred volts is the basis of its operation.

The image pick-up tube of this invention also employs a photoconductivelayer as a photosensitive layer, but a high speed electron beam of morethan several kilovolts to several tens kilovolts is used as a scanningbeam. Moreover, said scanning high velocity beam is not used fordischarging a charge accumulated on a photoconductive sensitive layerwhose quantity corresponds to the image as in a conventional vidicon,but the high velocity beam passes through a low density porousphotoconductive film which is a photosensitive target characteristic ofthis invention and produces secondary electrons to provide currentmultiplication when passing. Said electrons are collected with acollector and thus a television image signal is obtained.

The objects of the invention generally set forth, together withancillary advantages, are attained by the construction and arrangementshown by way of illustration in the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the structure and the principle ofoperation of an image pick-up tube according to an embodiment of thisinvention;

FIG. 2 is a magnified composition diagram of a target shown in FIG. 1;

FIGS. 3a and 3b are magnified composition diagram of a TSEM target foruse in a pick-up tube of this invention;

FIG. 4 is a sectional diagram showing the state wherein electrons aremultiplied at the photoconductive TSEM layer;

FIG. 5 is a cross-sectional view of TSEM photoconductive target whereintwo kinds of materials compose a mixed layer intertwined with the porouslayer; and

FIG. 6 is a cross-sectional view of a porous TSEM target according toanother embodiment of this invention.

This invention is now described in detail with reference to FIG. 1. Thereference numeral 1 indicates a photosensitive surface of the targetmade of a low density photoconductive film characteristic of thisinvention, 2 designates an electron gun projecting an electron beam ofmore than several kilovolts onto the target 1 with a high resolution and3 shows a face for introducing an image from a face in front of the tubeinto the target 1 the inner surfaces of which are coated with atransparent conductive treated surface or a NESA film 3. Referencenumeral 4 designates a lens system for electromagnetically focusing theelectron beam, 5 designates a high energy focused electron beam emittedfrom the electron gun 2, 6 indicates a glass tube and 7 indicates anoptical lens system for focusing a picture to be picked-up into an imageon the target 1. During ordinary performance, the target 1 is grounded,the electron gun 2 is provided with a high negative potential and a highenergy electron beam is made to collide with the target 1 by use of theelectron beam 5. A suitable positive potential is applied to the NESAfilm 3 with respect to the target 1 through a resistor R. Now, themechanism of the sensitive photoelectric conversion of the target 1 as apick-up tube will be explained. FIG. 2 shows a magnified compositiondiagram of the target 1. The target 1 consists of a metal film 8 and adeposited low density insulating layer 9 of quite fine photoconductiveparticles as shown in the figure. Generally, in the case of aconventional transmission secondary electron multiplication device, whena high energy electron from electron gun 2 is injected, as shown in thedrawings, from the side of the metal film into the target formed bydepositing insulators like KCl or other alkali halides, etc. on themetal film support member in a way to make a porous low densityinsulating layer, the high energy electron transmits through the metalfilm, enters the porous low-density insulating layer, undergoes multiplereflection therein and produces secondary electrons multiplied ingeometrical progression. As a result, a large number of secondaryelectrons are emitted from the back surface of the low densityinsulating layer as shown by arrows 12. Such a phenomenon is known astransmission secondary electron multiplication (hereafter denoted asTSEM).

This invention intends to provide a novel pick-up tube wherein amaterial producing said TSEM is used as a photoconductive material andthe secondary electron multiplication is controlled by external light.TSEM is based on the multiplication of secondary electrons ingeometrical progression produced when a primary electron is injected ona low density porous insulating layer as described hereinabove, but ithas no photoelectric sensitivity and the photoelectric conversion effectthereof has not been utilized. According to this invention, the controleffect due to the external light is added to the TSEM effect of aphotoconductive body at a porous layer by forming a low density porousphotoconductive layer of a high resistance photoconductive materialhaving a very small dark current like CdS, CdSe, Sb S etc. instead ofsaid low density porous insulating material. According to theconventional TSEM effect of an ordinary porous insulator, a large numberof positive charges are left on the surface of the insulator inside theporous space'of the low density insulating layer of FIG. 2. In thegenerating process of the secondary electron within the low densityinsulating layer a progressive generation of the secondary electron istaking place in the porous insulating layer as said positive chargesappear on the insulator surface producing an accelerating electric fieldand further in turn contribute to the generation of the subsequentsecondary electrons emission. When the secondary electrons 12 generatedin this way are emitted in a right hand direction from the surface ofthe porous low density insulating layer, positive charges correspondingto the quantity of the emitted secondary electrons are left on thesurface of the layer and this time these work to inhibit the emission ofsecondary electrons from the surface. Accordingly, when a suitablepositive potential is applied to the collector electrode 3 so as tocapture the emitted secondary electrons, the surface potential of thelow density insulating layer becomes equal to the potential of thecollector electrode and the secondary electrons are continuously emittedfrom the surface until equilibrium is reached if the insulating propertyof the layer is good.

The quantity of the finally emitted secondary electrons 12 depends uponaccelerating voltage, current density of the primary electron beam 5incident on the metal film of the target, the material, structure andthickness of the low density insulating layer 9, the distance betweenthe collector electrode 3 and the secondary electron emission surface,the potential of the target electrode 3 and the like and the gain of theTSEM target becomes several tens to several hundreds. The aim of thisinvention is to provide a photoconductive TSEM target, wherein the gainof the secondary electron emission can be controlled by the externallight 13, by using a photoconductive material having quite a high darkresistance instead of the ordinary insulator materials performing TSEMoperation as described hereinabove and to utilize said TSEM target as animage pick-up device.

Namely, the porous photoconductive layer 9 is composed of aphotoconductive body material with a higher resistivity, and the SnOfilm (UESA film) 3 formed on the inner surface of the window glass plateforms a collector electrode which captures the transmitted secondaryelectrons emitted from the surface of the low density photoconductiveporous layer 9 when a suitable positive potential is applied to the film3.

The object 10 to be picked up is projected on the surface of the lowdensity photoconductive layer 9 as shown at 11 by the lens system 7through the window glass 6 and the transparent conductive layer 3.

Said low density photoconductive layer has a thickness of 40,u, butbecause of its low density the incident light penetrates inside andexhibits photoconductivity corresponding to the quantity of the incidentlight.

When the high velocity electron beam 5 is made to impinge on the lowdensity photoconductive layer through the metal film 8, multipliedsecondary electrons are generated at the part where the beam hits due tothe TSEM effect and turn into a collector current at the collector 3.

As has been described hereinabove, the gain of the photoconductive TSEMlowers according to the electrical resistance of the low densityphotoconductive layer 9 because the influence of the positive chargeleft by the secondary electron emission and thus the output current ofthe secondary electron in this case becomes inversely proportional tothe light intensity of the image projected onto the surface of the lowdensity photoconductive layer. Accordingly, if this light image 11 isfocused on the surface of the low density photoconductive layer 9, theconductivity of the target depends on the brightness of the image 11.The primary electron beam 5 is scanned by the television scanning system4 as shown in FIG. 1 and a suitable positive potential is applied to thecollector electrode 3 composed of NESA film. The multiplied transmissionsecondary electron current corresponding to the brightness of the image11 flows through the load resistance R provided at the collector and animage signal can be taken out by way of a capacitor C.

As has become apparent from the foregoing descrip tion, the deviceaccording to this invention comprises a low density porousphotoconductive layer exhibiting conductivity corresponding to theincident light image and means for scanning said photoconductive layerby a high energy electron beam, and in this device the effect oftransmission secondary electron multiplication is controlled by theincident light. According to this device, not only a quite highresolution can be obtained but also a large output can be derived easilyby use of the transmission secondary multiplication effect in the lowdensity layer of the photoconductive material.

Further, a target having a porous low density spongelike structure canbe fabricated easily, e.g., by evaporation in low pressure (belowseveral mm. Hg) inert gas.

Now, another form of photoconductive TSEM target for use in a pick-uptube of this invention will be described.

For example, the target 1 consists of a fine metal mesh 81 and a poroussponge-like deposited layer 9 formed of fine particles of aphotoconductive material as shown in FIGS. 3a and 312. Since thephotoconductive TSEM target previously shown in FIG. 2 is formed in away that a metal film like aluminum film is used as a support member andthat a porous low density layer is formed thereupon, a high velocityelectron beam of at least more than several kv. is required to penetratethrough the metal film which works as a support member.

In the embodiment shown in FIGS. 3a and 3b, a metal layer having holeslike a fine metal mesh is used as the support layer in place of themetal film in order to make it unnecessary to give the primary electronan energy high enough to transmit through the conventional metal film orthe support layer. When the electron beam 5 is made to impinge on thelow density photoconductive layer 9 through the fine metal mesh 81,multiplied secondary electrons are generated at the part where theelectron beam is injected on account of the TSEM effect and they turn toa collector current of the opposing collector.

Thus, the image pick-up tube of this invention is similar toconventional ones in that a photoconductive film is used as a sensitivelayer, but diiferent in the point that a scanning electron beam ofseveral tens to several hundred volts is used as a primary electron beamfor generating transmission secondary electrons at the photoconductiveTSEM target. The structure of the target includes two typical examplesshown in FIGS. 3a and 3b. Though the targets shown in FIGS. 3a and 3bare similar to each other, the grating holes of the support mesh 81 ofFIG. 3a are not packed with a low density porous photoconductivematerial Whereas these of the mesh of FIG. 3b are packed withphotoconductive material. The setting of the porous low densitysponge-like photoconductive material to the grating of the metal mesh isdone by setting collodion film, which can be burned out in oxygenatmosphere, like nitrocellulose, etc. to one side of the mesh,depositing a photoconductive material from either side in inert gasatmosphere like argon of several mm. Hg and burning collodion film inoxygen. When the photoconductive material is deposited on the mesh fromthe side not comprising the collodion film, the structure of the meshbecomes as shown in FIG. 3a and when the evaporation is done from theside comprising the film, the structure as shown in FIG. 3b is provided.

FIG. 4 is a sectional diagram showing the state wherein electrons aremultiplied at the photoconductive TSEM layer and turned into emittedsecondary electrons 12 when the primary electron beam 5 impinges on themesh target from the left.

In a photoconductive transmission secondary electron multiplicationtarget of such a structure, a resolution of several to several tens oflines per mm. is possible when a suitably fine mesh is used and thetarget can be used not only for simple transmission secondary electronmultiplication (TSEM), but also for a high resolution image pick-up tubeutilizing a photoconductive secondary electron target.

When an output sensitivity of such a pick-up tube using a TSEM target isto be increased, the multiplication effect of the transmission electronsat the porous low density photoconductive layer becomes an importantproblem. When photoelectric sensitivity is not taken into account, KCl,other alkali halides or other metal oxide insulators are usually used asthe porous layer material for the ordinary transmission secondaryelectron multiplication both because the secondary electron emissionrate of these materials, 5, is approximately equal to -6 and remarkablyhigh and because they have a good insulating property. On the otherhand, the TSEM target is formed of various photoconductive materialslike CdS and it has an intrinsic resistance or a dark resistance as wellas its photoconductive property in contrast to said insulator andmoreover the secondary electron emission rate is not so high. Therefore,the gain of TSEM is not sufficient if the TSEM layer is formed only ofphotoconductive materials. In such a case, it is necessary to increasethe secondary electron multiplication effect by some means and at thesame time to provide a porous TSEM layer having a high photoconductivesensitivity.

For such a purpose, the sponge-like porous layer is formed by usingalkali halides like KCl or other metal oxides in combination withphotoconductive materials to enhance the dark current as well asphotoconductivity and the transmission secondary electron multiplicationfactor. These two kinds of materials compose a mixed layer intertwinedwith the porous layer as shown in FIG. 5. FIG. 5 shows a cross-sectionof such a TSEM photoconductive target, wherein 8 indicates a supportfilm supporting the porous layer which consists of metal, semiconductor,one kind of insulator or the combination thereof, the black particles 92indicate particles of photoconductive materials for forming the porouslayer, 93 designates particles of alkali halides like KCl or other metaloxides or the mixture thereof, 5 designates a primary electron beamhaving a high acceleration voltage sufiicient to transmit through thesupport film layer and 12 shows the transmission secondary electrons.Such a porous low density sponge-like layer formed of the mixture ofphotoconductive materials and insulators like alkali halides can befabricated by simultaneously depositing said two kinds of materials invacuum of less than several mm. Hg or inert gas like argon. When therate of TSEM is to be enhanced, the rate of combination of thephotoconductive material and the insulator, the deposition of speed ofthe two, the method of composing a porous layer, the method of heattreatment after deposition etc. must be suitably selected.

. FIG. 6 shows a cross-section of a porous TSEM target according toanother embodiment of this invention, which is formed into sandwichedlaminated layers consisting of photoconductive particles 92 andinsulating particles 93 by depositing photoconductive materials andinsulators alternatively in low pressure inert gases.

In such a target, the porous low density layer is formed by mixingphotoconductive materials with materials having a high secondaryelectron emission rate and using layer distribution. In such a device,the transmission sec ondary electron multiplication gain increasesremarkably and an image pick-up tube of this invention which has a highphotoelectric sensitivity can be obtained.

Further, in such a case where a photoconductive material is mixed with amaterial having a high secondary electron emission rate and a layerdistribution thereof is employed, a support member consisting of a metalmesh as described hereinabove can be used as the support layer insteadof the metal film.

What is claimed is:

1. An image pick-up tube having at one end a window for receiving lightimages and at the other end an electron gun for providing a scanningbeam of high speed electrons, a transparent collector on the inner faceof said window, a target spaced from said collector, said targetconsisting of a metal support member provided with a porousphotosensitive layer of material exhibiting transmission secondaryelectron multiplication when struck by haid high speed electron scanningbeam from said electron gun, said collector facing the porousphotosensitive layer to receive therefrom the transmitted m-ultipliedsecondary electrons from said porous layer.

2. A pick-up tube according to claim 1, further comprising a targethaving a metal support member having holes like a fine metal mesh and aporous photoconductive layer exhibiting conductivity corresponding tothe incident image.

3. A pick-up tube according to claim 2, wherein a part of said porousphotoconductive layer is packed into the holes of said metal supportmember.

4. A pick-up tube according to claim 1, further comprising aphotoconductive layer including a photoconductive material and at leasta kind of material having a high secondary electron emission rate likealkali halide.

5. A pick-up tube according to claim 2, further comprising aphotoconductive layer including a photoconductive material and at leastone kind of material having a high electron emission rate like alkalihalide.

References Cited UNITED STATES PATENTS 2,579,772 12/1951 Wilder 117-2102,683,832 7/1954 Edwards et al. 313- 2,910,602 10/1959 Lubszynski et al.313-65 3,128,406 4/1964 Goetze et al 313-65 3,148,297 9/1964Schneeberger et al. 313-65 3,213,315 10/1965 Lempert 313-65 JAMES W.LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner US. Cl. X.R.

